The complete primary structure of LDH-A4 isozymes from human and mouse has been determined by sequence analyses of both proteins and cDNA clones. Recently, the cancer-associated LDH-K isozyme induced by Kirsten sarcoma virus has been shown to be a modified form of human LDH-A4 protein complexed with ras P21 protein. Bovine LDH-A4 isozyme was shown to be phosphorylated in vitro by src tyrosine kinase. The low-salt eluting ssDNA binding protein of 36,000 daltons from mouse myeloma was shown to exhibit LDH enzymatic activity and cross-reacted immunologically with LDH-A4 isozymes from mouse, bovine and human. The LDH-A4 isozymes purified from human and bovine were also shown to bind to ssDNA on Western-blot and filter-binding assay. Human and bovine LDH-B4 isozymes showed only weak ssDNA-binding.